RF and microwave applications benefit greatly from the use of tunable devices and circuits. With components that can be tuned over a broad range, filters can be made to tune over multiple frequency bands of operation, impedance matching networks can be adjusted for amplifier power level or antenna impedance optimization. In general, tunability in these circuits gives the designer an additional tool in meeting the stringent frequency and power requirements of wireless communications systems, even in the changing operating environment inherent in these systems. In addition, tunable circuit responses can be made to compensate for the deleterious effects of aging and temperature changes in sensitive RF circuits. As the communications spectrum becomes more crowded, the demands on battery life, size and weight constraints intensify, and cost pressures increase, in which case technologies that enable such adaptability become increasingly important.


Barium strontium titanate, often referred to as BST, is a ferroelectric ceramic that exhibits an electric field-dependent dielectric constant. Capacitors fabricated with BST as the dielectric material therefore have capacitance values that can be adjusted by applying a DC voltage across them. In the equation for the capacitance of a parallel-plate capacitor, C = 0rA/t, it is the relative dielectric constant, er, that is varied in a BST capacitor, and can be typically varied by a 2:1 ratio or greater. Other candidate technologies that have been investigated and used in tunable circuits are semiconductor varactor diodes and MEMS capacitor structures. Both of these device types vary capacitance by adjustment of the dielectric thickness, t, rather than dielectric constant. Because of its voltage tunability and high value of dielectric constant, high power handling capability, fast tuning response and ease of integration with other thin film components, MIM capacitors, based on thin film BST, present an attractive technology for the design of tunable circuits.

BST Material Characteristics

Ferroelectric materials have received a great deal of attention from the dynamic random access memory (DRAM) industry in the last few decades in the drive toward gigabit density memories. Critical for high density memory cells is high capacitance, and a high dielectric constant capacitor provides a small-sized alternative to simply increasing the area or decreasing the thickness of existing capacitor technologies. BST in particular has been studied because of its high dielectric constant, good temperature characteristics, low loss and potential for device integration.1 A number of different film deposition techniques, film thicknesses, film compositions and electrode materials have been characterized in detail, and the results can be found in the literature.2,3 This article focuses on the properties and applications of thin film (typically below 500 nm) BST layers deposited by sputtering techniques and using platinum as the electrode material, since this structure has been found to have among the most promising characteristics for RF applications.

BST is in the class of crystalline materials referred to as ferroelectrics, implying that its spontaneous polarization (without the application of an electric field) is reversible or re-orientable. Incidentally, ferroelectrics also have the property of piezoelectricity (stress induced polarization) and pyroelectricity (temperature induced polarization), but those properties are not important to this discussion.

Fig. 1 Ferroelectric and paraelectric phases of BST.

Most ferroelectrics including BST will, when the temperature reaches the Curie point, Tc, undergo a phase transition from the ferroelectric to the paraelectric state. This is an important property. Below the Curie point, the material is in the ferroelectric phase, its lattice structure is tetragonal and its polarization response to an applied electric field is hysteretic, as indicated in Figure 1. When the Curie point is reached the material undergoes a transition to the paraelectric phase, the crystal lattice changes from tetragonal to cubic and it displays the highly desirable nonhysteretic electric field response that is also shown. Another important phenomenon related to this phase change is an increase in the temperature dependence of the relative permittivity, or dielectric constant, in the neighborhood of Tc, as shown in Figure 2.

Fig. 2 Temperature dependence of the relative permeability of thick and thin films of BST.

The strong temperature dependence of the dielectric constant occurs around the Curie point, which unfortunately happens to be near room temperature. However, this behavior applies only to thick film BST, where the film thickness is greater than about 500 nm (0.5 µm). In the thin film regime, the material remains in the paraelectric state, its polarization response is nonhysteretic and the temperature dependence of dielectric constant is greatly suppressed. The BST’s dielectric constant in thin film form is generally in the 200 to 300 range, yielding high capacitance densities and therefore relatively small-sized capacitors. Film thicknesses in this range are also compatible with standard thin film processing, meaning that integration of BST capacitors with other thin film components is possible. For all these reasons, it is extremely desirable to use BST in thin film form.

BST, with its chemical formula of (BaxSr1-x)TiO3, can be thought of as a solid solution of barium titanate, BaTiO3, and strontium titanate, SrTiO3. By adjusting the Ba/Sr ratio and other film growth parameters, precise control can be exercised over the dielectric constant, tunability, ∆C/∆V and quality factor Q of the resulting capacitors.

The BST Capacitor

With the basic material properties of barium strontium titanate understood, the operation of the tunable capacitor using BST as the dielectric can easily be appreciated. With commonly used values of film thickness and compositional parameters, a 3 pF capacitor, for example, could be fabricated in a parallel-plate structure with a plate area of approximately 300 µm2. Using the bias-T or resistor biasing arrangement shown in Figure 3, a DC voltage can be applied across the BST capacitor, and its capacitance value will change in response to the field appearing across it. A typical capacitance vs. tuning voltage curve appears, as shown in Figure 4, with the capacitance reduced by half, to 1.5 pF, at approximately Vtune = 5 V, and by two-thirds, to 1 pF, at about 10 V. The actual tuning response of the capacitor can be adjusted by varying the dielectric thickness and other growth parameters.

Fig. 3 Simple biasing circuit for a tunable BST capacitor.

The breakdown voltage of this film is greater than 20 V so that substantial RF voltage swings can be tolerated, even when the maximum tuning voltage is applied. If more RF voltage swing is required, a lower tuning voltage can be used or multiple BST capacitors can be placed in series, effectively dividing the voltage swing among them. The latter approach also improves the linearity of the resulting circuit.

A key parameter of the circuit elements used in RF applications is the device Q, or quality factor, because it affects loss and noise characteristics of the circuits. As the ratio of energy stored to energy dissipated, Q is a function of the series resistance Rs intrinsic to the device and the loss tangent tan d of the dielectric. The material loss tangent dominates the lower frequency Q value, with Rs causing decreasing Q values for higher frequencies. As a simple metal-insulator-metal (MIM) structure, BST capacitors minimize Rs and maintain relatively good Qs at RF frequencies. In contrast, varactor diodes must also conduct current through an undepleted semiconductor region, an additional source of Rs in the device, causing the Q to fall off more quickly with frequency. The result is that although varactor diodes can have quite high values of Q at low frequencies (usually quoted at 50 MHz), BST-based variable capacitors generally have higher Q values at RF frequencies. For BST capacitors, Q values at 2 GHz are generally in the 50 to 100 range for capacitor values of 1 to 5 pF.

Fig. 4 Capacitance vs. tuning voltage.

Tunable Filter Application

A tunable capacitor in an L-C resonant circuit can be used to tune the resonant frequency. As an extension of this, when used in filters, capacitance tuning effects a change in the frequency response of the filter. Frequency agile filters may enable the designer to replace two or more fixed value filters, or may enable more precise tuning of a fixed filter. An example of a tunable bandpass filter response is shown in Figure 5 for a two-pole filter utilizing two tunable 1 pF BST capacitors. The center frequency of the filter changes from 0.91 to 1.23 GHz (~35 percent increase) with a 50 percent reduction in capacitance. Additional frequency shift can be achieved with increased capacitor tuning.

Fig. 5 Tunable two-pole filter response as a function of tuning voltage.

The insertion loss of this filter is in the 6 to 7 dB range, primarily due to the effects of the inductor Q. To meet filter insertion loss requirements that are more stringent than this, an alternative is to use the tunable BST capacitor to add frequency tuning to an existing high Q filter. An example would be adding the tunable capacitor in shunt with the capacitance of a high Q ceramic monoblock resonator filter. A sacrifice in tuning range of the filter is expected since the total filter capacitance is composed of the ceramic monoblock’s capacitance plus that of the BST capacitor itself, and only the BST capacitance is tuned.

Fig. 6 Tunable ceramic monoblock filter.

Measured results for such a design are shown in Figure 6. At zero bias on the BST capacitor, C = C0, and the center frequency, f0, of the filter is 913 MHz. When Vtune is applied such that C = C0/2, f0 becomes 977 MHz for a tuning range of 64 MHz, or an increase of 7 percent. The Q value of the filter as calculated from f0 and the 3 dB bandwidth starts at 135 and increases to 217 at the high frequency. The data shown was taken with the measurement system lightly coupled to the monoblock. This allows an accurate determination of the filter Q and frequency response, but causes an artificially high insertion loss. Using this tuned monoblock approach an insertion loss less than 2 dB is possible. By adjusting the value of the BST tuning capacitor in relation to the monoblock’s intrinsic capacitance, the optimum trade-off between frequency tuning range and insertion loss can be chosen.

Tunable Matching Network Application

Another application for tunable capacitors is in matching networks for amplifiers and antennas. The air interface in wireless networks presents a changing environment in which amplifiers and antennas must propagate signals as efficiently as possible. Fixed matching networks are designed for optimal match at a specific frequency or amplifier output impedance, but power transfer efficiency will decrease as the frequency or output impedance changes from the design bandwidth values.

In many cases, a frequency band of operation will be divided into a number of narrower channels to be used by individual users. The equipment must be able to function in all of the channels at any given time, but will only operate in one of them at a time. A broadband matching network can be designed to match over all the channels simultaneously and will have an insertion loss that reflects this bandwidth. The Bode-Fano criterion implies that by narrowing the bandwidth of the matching network a lower insertion loss can be achieved. Tunable matching networks take advantage of this by providing a low insertion loss, narrow band match that can be frequency adjusted as needed.

Fig. 7 Response of a tunable antenna matching network.

Figure 7 shows the measured performance data for just such a tunable matching network design when tuned to three different frequencies. In this case, antenna matching is required over approximately 425 to 490 MHz. A broadband matching network design covering the entire band yielded an insertion loss of approximately 1.5 dB, or approximately 70 percent efficiency in transferring power to the load. The narrow band, tunable match provides an insertion loss of 0.6 dB or better, corresponding to at least 87 percent power transfer efficiency. Figure 8 shows the circuit schematic of the tunable network, which uses two tunable BST capacitors, C1 and C2, and two corresponding control voltages, V1 and V2.

Fig. 8 Tunable matching network.

With handset power at a premium due to battery life limitations, tunable matching networks can provide a valuable improvement in the efficiency of power use, and efficiency enhancement becomes especially important at the output of the transmitter power amplifier, where 50 percent or more of the battery’s power may be consumed.

Other uses for tunable matching networks include interstage matching for power amplifiers, effectively broadening the bandwidth of the device, and output matching of the amplifier that is used to enhance power-added efficiency (PAE) as the amplifier’s output impedance changes with output power level.

Phase Shifter Application

One final application for BST technology that deserves mention is phase shifters.4 Phase shifting devices have several applications in wireless communications systems — “smart antennas” based on phased arrays, amplifier linearization and amplifier power combining.

The operating principle of a BST-based phase shifter is straightforward and is based on a variable delay line. A transmission line is commonly modeled as a lumped element circuit consisting of a distributed series inductance and a distributed shunt capacitance (if the distributed series resistance and shunt conductance is neglected, as is usually possible). The phase change from input to output of the line is

Tunable BST capacitors are placed periodically along the length of the transmission line, capacitively loading the line, and the phase shift becomes

Since CBST can be controlled, the precise amount of phase shift can also be controlled. This situation is depicted schematically in Figure 9. The device is designed so that a single control voltage applied to one side of the line is all that is required to control all of the capacitors and therefore the phase shift. This is an important distinction from digital phase shifters that commonly require three to six control lines routed to each phase shifter. In large arrays this can amount to a significant control plane routing challenge.

Fig. 9 Variable delay line implementation of a phase shifter.

In this type of design each L-C section contributes a number of degrees of maximum phase shift with applied voltage, so adding more sections has the effect of adding to the total achievable phase shift. However, each section also contributes a certain amount of insertion loss, so the number of sections should be kept to the minimum necessary for the required amount of phase shift. Measured values of phase shift and insertion loss as a function of frequency and control voltage for a 12 GHz phase shifter are shown in Figure 10. This particular design requires 20 V of control voltage to achieve 360° of phase shift at 12 GHz and higher.

Fig. 10 S21 phase and magnitude for a 12 GHz phase shifter.

When compared to other phase shifter technologies such as GaAs and MEMS, the desirable characteristics possessed by BST-based phase shifters are a single analog control voltage, reasonable loss characteristics, negligible power consumption, high power handling, small size, high reliability and low cost.

Conclusion

This article has introduced the basic properties of thin film ferroelectric barium strontium titanate and explained how these properties can be exploited to fabricate a tunable capacitor, or varactor. The usefulness of this basic device was illustrated with several relevant design examples with the result that circuit designers involved primarily in wireless design should find this technology especially useful. Several current and important design challenges can be effectively met with this technology, allowing the wireless industry to continue the trends of smaller, lighter, more functional and less expensive equipment design.

References

  1. S. Ezhilvalavan and T.Y. Tseng, “Progress in the Development of (Ba,Sr)TiO3 (BST) Thin Films for Gigabit Era DRAMs,” J Materials Chemistry and Physics 65, 2000, pp. 227–248.
  2. D.E. Kotecki, J.D. Baniecki, H. Shen, R.B. Laibowitz, K.L. Saenger, J.J. Lian, T.M. Shaw, S.D. Athavale, C. Cabral, Jr., R.R. Duncombe, M. Gutsche, G. Kunkel, Y.J. Park, Y.Y. Wang and R. Wise, “(Ba,Sr)TiO3 Dielectrics for Future Stacked-capacitor DRAM,” IBM J. Res. Develop., Vol. 43, No. 3, May 1999.
  3. D. Damjanovic, “Ferroelectric, Dielectric and Piezoelectric Properties of Ferroelectric Thin Films and Ceramics,” Rep. Prog. Phys. 61, 1998, pp. 1267–1324.
  4. B. Acikel, T.R. Taylor, P.J. Hansen, J.S. Speck and R.A. York, “A New High Performance Phase Shifter Using BaxSr1-xTiO3 Thin Films,” IEEE Microwave and Wireless Components Letters, Vol. 12, No. 7, July 2002.

Bud Noren earned his BS degree in physics from Boston College and his MS degree in electrical engineering from Purdue University. He worked with Agilent from 1986 to 1999 in process/device engineering and marketing and, most recently, at Vitesse Semiconductor Corp., from 1999 to 2002, as a product line manager for SerDes Components. He joined Agile Materials & Technologies in 2002 and is currently director of marketing.